Electrode rapping control for an electrostatic precipitator



Jan. 2, 1968 I L, GLAESER .'3,360,902

ELECTRODE RAPPING CONTROL FOR AN ELECTROSTATIC PRECIPITATOR Filed April 20, 1965 v I 2 Sheets-Sheet 1 4/ v 4/ PI" if I C L FIG.

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ELECTRODE RAPPING CONTROL FOR AN ELECTROSTATIC PRECIPITATOR Filed April 20, 1965 I 2 Sheets-Sheet 2 1 CURRENT EL ECTE/CAL 69 ss/vsuva ENE/2m DEV/CE CONTROL 35 cue/25w CONT/20L H & 2325255 Fla. 2

, RELAY #66 c/ecu/r AAPPER TIMER CIRCUIT apps? v T L AP/ 52 INVENTOR. F/G 3 MELVIN L. 6LAE$EE BY (36 ALT Jim/1g 3,360,902 ELECTRODE RAPPING CONTROL FOR AN ELECTROSTATIC PRECIPITATOR Melvin L. Glaeser, Baltimore, Md., assignor to Koppers Company, Inc, a corporation of Delaware Filed Apr. 20, 1965, Ser. No. 449,569 14 Claims. (CI. 55-13) This invention relates generally to electrostatic precipitators and more particularly to the controlling of cyclic rapping of the electrodes within the precipitator.

In conventional electrostatic precipitators having means for directing a flow of gas containing entrained particles of matter past a plurality of electrodes, a high voltage direct current is applied to the discharge electrodes which current then flows to the collector electrodes in the form of a corona discharge. The corona discharges charges the particles of matter entrained in the gas flowing past the electrodes and the charged particles are attracted to and collect on the collector electrodes. Periodically the particles are dislodged and collected. Conventionally, the particles are dislodged by vibrating or, as it is called, rapping the electrodes.

Rapping of the electrodes causes the particles collected on the collector electrode to fall from the collector electrode into a hopper or the like. A small amount of reentrainment of the particles tends to occur. So long as the voltage applied to the discharge electrodes remains at a high level, any of the particles tending to be re-entrained are recollected on the collector electrodes at a point below the location where they were originally dislodged. This occurs until the particles are finally collected in a hopper or the like.

The foregoing procedure works well so long as a normal steady state condition exists in the precipitator. Conditions in a precipitator vary widely and suddenly, however. Because of the high voltage level of the discharge electrodes, dust concentrations in the gas, gas composition, water vapor content and the like, the gaseous resistance between the electrodes sometimes breaks down completely and this condition is manifested as a spark. The result of this is a voltage loss in the discharge electrodes and no dust collection occurs at this time. It has been found that sparking can be tolerated at the rate of about 100 sparks per minute but exceeding this results in decreased percipitator efiiciency. Thus, controls have been developed which automatically lower the voltage applied to the discharge electrodes to maintain the spark rate within acceptable limits. One such control is described in J. B. Thomas et al. Patent No. 2,961,577, assigned to the assignee of the present invention.

A lowering of the voltage, for example, to prevent sparking, results in the particles being charged to a lesser degree. If rapping occurs during a lowered voltage period, the particles dislodged during the period of reduced voltage tend to remain re-entrained in the gas so that the collection efiiciency is reduced. In addition, the re-entrained particles may cause even more sparking and enhance the dilficulty.

conventionally, rapping of the electrodes has been carried out independently of the control of electrode voltage. This is partly attributable to the fact that in numerous applications the operating conditions, within the precipiator remain reasonably constant so that no great problem results from the conventional mode of operation. However, the application of precipitators to the treatment of gases where operating conditions fluctuate widely, for example, where particles are removed from gases produced by open hearth or basic oxygen furnaces and similar applications has made the problem more acute. The practice heretofore has been to provide a precipitator of a size A United States Patent Patented Jan. 2, 1968 capable of efiiciently cleaning the gas under the worst possible operating conditions. This, of course, necessitates that the precipitator be unduly large and expensive.

Accordingly, an object of the present invention is to provide a system for stopping the rapping of the electrodes during periods of severe sparking and consequent reduced voltage in the discharge electrodes.

Another object of this invention is to provide rapping controls to increase the efiiciency of the precipitator during adverse operating conditions thus permitting the precipitator to be designedto meet the requirements of less operating severity.

This invention contemplates the provision of means actuated when the excitation to an electrode drops below a preselected value in response, for example, to increased sparkings to render inelfective the rappers, for example, to de-energize the power supply for the electrode rappers should the excitation remain at a low level beyond a desired length of the time. Should the excitation rise during the delay period, the rappers will not be rendered ineffective. Once rendered ineffective the rappers will not be rendered efiective until the efficiency of the precipitator is increased to its normal condition for the desired time.

The above and further objects and novel features of the invention will appear more fully from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are not intended as a definition of the invention but are for the purpose of illustration only.

In the drawings wherein like parts are marked alike:

FIGURE 1 schematically shows an elevation of the general arrangement of the electrode structure of an electrostatic precipitator facing in the direction of gas flow;

FIGURE 2 schematically illustrates the excitation and control for the precipitator of FIG. 1;

FIGURE 3 illustrates schematically by block diagram an electrical circuit in accordance with the invention for controlling the amount of rapping during periods of severe sparking of the precipitator of FIG. 1;

FIGURE 4 illustrates by detailed circuit an embodiment of the invention.

Referring now to FIG. 1, there is illustrated an electrostatic precipitator generally designated by the numeral 20. An outer shell 28 directs the flow of gases past discharge electrodes 31 and collector electrodes 35. The discharge electrodes are supported by structural members 37 which are themselves supported by hanger rods 39 extend ing through insulators 41 which electrically isolate the electrodes 31 from the shell 28 and the collector electrodes 35.

Referring to FIG. 2, the electrical precipitator is represented schematically as having a source 40 for supplying alternating current, a transformer 42 for raising the voltage level of the alternating current, a rectifier 43 for changing the alternating current to direct current, a discharge electrode 31 for charging the particles, a collecting electrode 35 for collecting the particles, a current control circuit generally designated as 44 for maintaining a constant current flow to the discharge electrode, and a spark-rate control system generally designated as 45 for regulating the voltage in accordance with a predetermined spark rate.

As the gas, bearing suspended particles of matter, passes through shell 28, the particles are charged by electrode 31 and deposited principally on the surface of electrode 35. The material collected on electrode 35 is removed by rapping or vibration at selected intervals.

The characteristics of the corona may vary with changes in the gas concentration and composition. These changes may greatly affect the power input to the precipitator. The power input is controlled to obtain maximum efliciency by the current control circuit 44 which is operative to maintain the current flow at a preset value in the absence of sparking, and spark-rate control circuit 45 which is operative to regulate the voltage at sparking.

The changes in normal current flow and at spark-over are sensed by the current sensing device 49. The changes of current flow caused by normal variation in corona discharge are reflected in gradual variations in the current flow from the preset value. These gradual variations in current are in contrast to the transient surges of current occurring at sparkover which result in substantially instantaneous increase in current flow usually of greater 'magnitude than the current flow variation during corona discharge.

The foregoing portion of the precipitator is conventional, and is described in detail in the aforementioned Patent No. 2,961,577.

A rapping source 56 is connected to the hanger rod 39 so that vibratory motion is transmitted through the structural member 37 and into the electrode 31. A suitable rapping means is disclosed in the patent to John W. Pennington No. 3,030,753. The collector electrodes 35 are supported by structural members 38 which are an integral part of the shell 28. The collector electrodes 35 are also rapped by a suitable vibrating source 59, similar to the rapping source 56, connected to the electrodes by a rod 60. Thus, when the rappers are energized, the electrodes are vibrated thereby dislodging accumulated dust which 1 falls into a hopper (not shown) or is otherwise suitably disposed of.

' Referring now to FIG. 3, an automatic power control unit 61 embodies elements 44 and 45 of FIG. 2, and includes a primary voltage circuit 62, a primary current circuit 63, a secondary voltage circuit 64 and a secondary current circuit 65. These circuits have voltage and current levels which are lowered in response to increased sparking between the electrodes. Any one of these voltage or current levels may be used to develop a sensing signal in accordance with this invetnion. This signal then operates a time delay circuit 66 interposed between the signal source and a rapper timer control 67. The rapper timer control 67 as is conventional energizes the rappers 56, 59 in a predetermined sequence for a predetermined time interval. Such controls are usually capable of being adjusted to vary the cycle and the time interval between rapping cycles as well as the time length of the cycles. Because control 67 is conventional, no further description is necessary for an understanding of this invention.

As has been pointed out before, the rapping of electrodes has heretofore taken place in a timed sequence and at intermittent time intervals. The disadvantage has been that the time intervals of rapping do not always coincide with the period of efficient operation of the precipitator. Consequently, there is a great tendency for the entrained particles to be carried out of the precipitator with the gas if the operating conditions during rapping are so adverse that sparking is occurring or that the voltage or current levels have been lowered so that the precipitator is not operating at its optimum efficiency. In accordance with this invention, the rapping of the precipitator is carired out during the normal steady state operation of the precipitator as it has heretofore, that is, by a cycle of operation which cycle of operation takes place over predetermined intervals of time; but inasmuch as adverse conditions are apt to occur at any time, provision is made for rendering the rapping system ineffective during the period that the precipitator is operating at a low level of efiiciency. Since the condition of low precipitator efliciency may be transient, this low efficiency of operation must persist of a period of time before the the rapping system is rendered ineffective. Similarly, after the rapping has been rendered ineffective and the rapping cycle interrupted and after the precipitator regains its normal operating efliciency, the rapping system is maintained ineffective until after the precipitator has operated at its normal efficiency for a predetermined period of time. This reduces or eliminates any tendency toward a hunting effect where the rapping system is rapidly shifting from an ineffective to effective condition over a short interval of time. The overall efliciency of the precipitator is maintained or increased because the particles which would have escaped with the gases during the rapping period at low efficiency in the arrangements heretofore known are retained in the unit. The retention of the collected particles during the period of time when adverse conditions are present and the precipitator is operated without its normal rapping period does not adversely affect the overall operation; the particles merely remain on the collector electrodes until such time as its normal operation is resumed and the particles are collected in the hopper by rapping.

Turning now to FIG. 3, there is schematically illustrated a monitoring circuit 66 for rendering ineffective the operation of the rappers of the electrodes during the time that severe sparking is occurring within the precipitator. Such adverse condition is reflected in parameters such as the voltage and current levels of the aforementioned circuits 63, 64 and 65 in the power control unit 61. At this adverse operating time, a level of a parameter will have changed from a normal level to a different level.

There is illustrated in FIG. 4 the sensing of a parameter by providing a means responsive to the existence of a low level of current for a predetermined period :of time for rendering the rapping means ineffective to operate. For example, if the parameter is below a preset level for a period exceeding a preset time the power supply to the rapper timing control 67 will be dc-energized.

For purpose of simplicity, the novel monitoring system is illustrated in FIG. 4 as being responsive to the current level supplied to the electrode. To this end, a current sensitive transformer 70 is provided for sensing the current flowing to the precipitator. The signal current induced in circuit 70 is fed into the operating coil 71 of a conventional current sensitive time delay device 75.

Conveniently, the delay device 75 may be current sensing relay such as the type COD sold by Westinghouse and described, for example, in US. Patent Nos. 2,697,187 and 2,488,443 and in Westinghouse Bulletin 4l-l00B, published September 1961.

Briefly, the relay has an induction disc unit, which embodies an E-type laminated magnetic structure. A main current coil (either tapped or untapped) is placed on the center leg of the magnetic structure. Flux produced by this coil returns through the two outer legs of the electromagnet. A shading coil on one of the outer legs creates an out of phase flux which reacts with the main coil flux to cause rotation of the disc in the air gap in the electromagnet, which rotation is opposed by a spiral spring on the disc shaft, which also carries the moving contact. Torque created by the electromagnet is balanced by the opposing torque of the spiral spring, and the disc shaft assembly with the moving contact assumes a position corresponding to the current applied to the electromagnet, unless the travel is limited by the setting of the stationary contacts. Disc rotation is damped by a horseshoe shaped permanent magnet.

In operation, when the magnitude of applied current is less than the setting of the low current stationary contact, the moving contact rests against this stationary contact. Applied current values between the high and low settings will cause the disc and moving contact to float between the high and low stationary contacts. Current values above the high current setting will cause the moving contact to close the high current circuit. A small change in current magnitude will cause the disc to move slowly from its original position to the new position. Conversely, a large change in applied current will move the disc at a faster rate. The positions of the stationary contacts, of course, have an effect on the speed of response. A close setting of these contacts will produce faster contact operation for the same change in current than does, a wide separation because the moving contact has farther to travel to make contact with a wide separation of the stationary contacts.

In this invention, the signal from current transformer 70 is applied to the operating coil of the relay and the adjustable high and low current closing contacts are adjusted to the desired time and current change to maintain the rappers in their normal cyclic and timed operation until the precipitator has operated at a lowered efiiciency for a sutticient length of time to justify the rappers being taken out of operation or rendered ineifective to operation.

Once the rappers have been rendered inetfective to operate, they will not, in accordance with this invention, be rendered eifective to operate until after the precipitator has reached a desired range of efficient operation for a desired period of time. Thus, the contacts of the delay switch are provided with circuits which maintain the rapper system either effective to operate or ineffective to operate. To this end, the fixed contacts 84 and 89 are con.- nected in parallel circuits between the excitation lead 80 and the ground lead 78 to energize respective relays 86 and 87. A holding circuit is provided for each relay so that once energized the relay will remain energized even though the fixed contact that originally energized the relay opens. Thus contact 84 is bridged by lead 90, normally open contact 91, lead 92, normally closed contact 93 and lead 94. Contact 89 is bridged by lead 95, normally closed contact 96, lead 97, normally open contact 98 and lead 99.

Upon energization of the system, excitation is applied through lead 80 normally closed contact 89 to relay 87 and the ground 78. Relay 87, upon thus being energized, closes the normally open contact 98 and thereby provides a circuit for maintaining relay 75 energized even though the contacts 89 may later be separated. The energization of relay 87 also opens the normally closed contact 93 thereby preventing de-energization of relay 86 through any circuit other than by the closing of contacts 84 so long as relay 87 is energized. The control system for the precipitator tends to bring the precipitator to an optimum voltage or current condition as rapidly as possible. Consequently, the disc of the delay device moves to the high contacts. Upon the discs reaching the high closing contacts, contacts 84 are engaged thus ener gizing relay 86. Upon the energization or relay 86, the normally closed contact 96 is disengaged. This opens the holding circuit for relay 87 and de-energizes relay 87. Thereupon, contact 93 returns to its normally closed position, as illustrated. The energization of relay 86 has closed the normally open contact 91 thereby rendering effective the holding circuit for relay 86 so relay 86 remains energized even though the contact 84 may be opened by subsequent less efficient operation of the precipitator. Due to this holding circuit, the relay 84 remains energized until such time as the induction disc reaches the low terminal at which time contacts 89 are closed and the holding circuit for relay 84 de-energized by the disengagement of contact 93.

The relays 86 and 87 control the rappers by way of contacts 110 and 111 relay solenoid 86 operating contact 110 and relay solenoid 87 operating contact 111. These contacts, as illustrated in FIG. 4, are connected in parallel between power lead 113 and connecting lead 115 to the rapper timer circuit 67 which operates rappers 56 and 59. When relay solenoid 87 is energized, contact 111 is open and when relay solenoid 84 is energized contact 110 is closed.

When the magnitude of the signal current, as sensed by sensor 70, is less than desired, the induction disc of device 75 is caused to rotate until contacts 89 are closed to energize the low current circuit. When the magnitude of the signal current is at or exceeds the desired value, the induction disc will rotate until contact 84- closes to close the high current circuit. Signal current valves between the high and low valves will cause the moving contacts to float between the high and low contacts 84 and 89. Relays 86 and 87 are provided to lock in either the high or low current circuits once the contacts 84 or 89 have been closed so that, for instance, if the high current circuit 5 is locked in and the signal current valve is decreased, the high current circuit will not open until the moving contact has closed the low current circuit. Since the stationary contacts 84, 89 may be positioned relative to each other, the time required for the moving contacts to move from 10 one to the other may be varied. Thus, the high current circuit will remain locked in unless the signal current falls below the low contact setting for a preselected time interval. Conversely, the low current circuit will remain locked in until the signal current exceeds the high contact setting for a preselected time.

By way of contact 110, the high current circuit is interposed in the power line permitting current to flow to the circuit 67 to operate the rappers in their programmed sequence. Should the signal current as rendered by sensor 7 0 decrease for a preselected time interval in response to operating conditions within the precipitator, then the high current circuit will open contact 110 thus preventing current fiow' to the rapper timer control. This condition will be maintained until the signal current again reaches a satisfactory level.

In operation, the device 75 is adjusted to be responsive to a sensing signal corresponding to a level of voltage potential in the discharge electrodes below which it is desired that rapping of the electrodes should not occur. The

relay is further adjusted for the time interval desired before which the rappers are deenergized after the sensing signal has dropped to the preset level. Thus, when the precipitator is operated in the conventional manner, if the operating characteristics change within the precipitator so that severe sparking occurs and the voltage potential is lowered by the automatic power control beyond the predetermined time interval, then the rappers will be deenergized. Consequently, rapping of the electrodes will cease so that collected dust will not be dislodged and reentrained in the gas during the period of severe sparking. Even though the voltage potential on the electrodes may not be increased to the extent necessary to reactivate the rappers, precipitator efliciency will be increased because of reduced re-entrainment of dust. When the adverse operating conditions pass, the voltage potential will consequently be increased and when the sensing signal reaches the proper level and remains there for a desired time the rappers will be reactivated and the precipitator will operate in the conventional manner. In this way, overall precipitator efliciency is maintained despite periodic intervals of adverse operating conditions.

Having thus described my invention in its best embodiment and mode of operation, what I desire to claim by Letters Patent is:

I claim:

1. In an electrostatic precipitator having a power supply which furnishes high voltage to the discharge electrodes responsive to operating conditions within the precipitator and a control means for sequencing rapper operation, the

6 method comprising:

directing the flow of gas to be cleaned through the precipitator,

supplying high voltage to the discharge electrodes of a magnitude sufiicient to cause collection of charged particles on the collector electrodes,

rapping the electrodes in a timed cycle to dislodge the collected particles,

collecting the dislodged particles, and

interrupting rapping cycle when adverse operating conditions occur Within the precipitator to reduce reintrainment in the gas stream of collected particles during said occurrences.

2. In an electrostatic precipitator having a power supply which furnishes high voltage to the discharge electrodes responsive to operating conditions within the precipitator and a control means for cyclic rapper operation, the method comprising:

directing the flow of gas to be cleaned through the precipitator,

supplying high voltage to the discharge electrodes of a magnitude Sllfl'lCifil'lt to cause collection of charged particles on the collector electrodes,

rapping the electrodes periodically to dislodge the collected particles,

collecting the dislodged particles, and

monitoring the current level to the electrodes, and

interrupting the rapping of the electrodes when said current level falls below a predetermined level so that reintrainment in the gas stream of collected particles is reduced during said low current levels.

3. The method of claim 2 wherein the interrupting of rapping of the electrodes takes place after the current has remained at a reduced level for a predetermined time.

4. The method of claim 2 wherein the rapping of electrodes is resumed after a predetermined elapsed time following an increase in current level.

5. In an electrostatic precipitator having discharge and collector electrodes whose differential current level is responsive to operating conditions within the precipitator,

means for rapping the collector electrodes to dislodge collected particles,

control means for said rapping means for operating said rapping means,

means responsive to adverse operating conditions within the precipitator, and

means operable by said responsive means for interrupting the cycle of rapping of the electrodes to prevent rapping of the electrodes during said adverse operating conditions.

6. An electrostatic precipitator comprising:

discharge and collector electrodes,

means to direct the flow of gas to the precipitator,

power supply means responsive to operating conditions within the precipitator to supply current to said discharge electrode,

means for rapping the collector electrode to dislodge collected particles,

means for collecting the dislodged particles, and

control means, interposed between and operably connected to said power supply and said rapping control means,v and responsive to the current supply to the discharge electrodes for interrupting the rapping of the electrodes during a low level of current supply.

7.'The apparatus of claim 6 including means operatively connected with said control means for maintaining the rapping of the electrodes interrupted for a predetermined elapsed time following a reduction in said curcent supply. v

8. The apparatus of claim 6 wherein the control means permits a resumption of electrode rapping after a predetermined elapsed time following an increase in said current level.

9. The apparatus of claim 7 wherein the control means is adjustable so as to vary the elapsed time after which rapping is prevented following a reduction in said current level.

be cleaned through 10. The apparatus of claim 8 wherein the control means is adjustable so as to vary the elapsed time after which rapping is permitted following an increase in said current level.

11. The apparatus of claim 6 wherein the control means is adjustable so as to vary the current level to which the control means is responsive.

12. The apparatus of claim 6 wherein the control means is a current-sensitive relay having a movable contact which assumes a position corresponding to the current applied and high current and low current closing contacts, independently adjustable in a manner to vary the elapsed time from a closed high contact position to a closed low contact position in response to a reduction in said current level.

13. The apparatus of claim 12 wherein, in addition, relay means are operably connected to the control means in a manner to maintain either of the said closing contacts closed until such time that the said movable contact closes the said closing contact not being maintained closed.

14. An electrostatic precipitator comprising:

a collector electrode,

a discharge electrode,

a source for energizing said electrodes whereby particles in gases passing in said electrode are charged by said discharge electrode and drift to and are deposited on said collector electrode,

a receiver,

a rapper for dislodging the particles deposited on said electrode whereby the particle falls to said receiver,

rapper control means for cyclically operating said rapper,

means responsive to the energization to said electrodes for rendering said rapper control means ineffective when the energization is below a normal level for a predetermined period of time, and

means for rendering said rapper control means effective after the energization returns to a normal level for a predetermined period of time.

References Cited UNITED STATES PATENTS 1,339,471 5/1920 Meston 55--1l2 2,756,839 7/1956 Roberts 55-1 10 2,854,089 9/1958 White et al 55-112 2,858,900 11/1958 Foley 55-112 2,943,697 7/1960 Little 55l05 2,961,577 ll/l960 Thomas et al. 55105 2,976,951 3/1961 Lagarias 551 12 X 2,978,065 4/1961 Berg 55--l10 X 3,030,753 4/1962 Pennington 55--112 3,039,252 6/1962 Guldtemond et al 55105 FOREIGN PATENTS 609,426 5/ 1926 France.

FRANK W. LUTTER, Primary Examiner.

D. TALBERT, Assistant Examiner. 

1. IN A ELECTROSTATIC PRECIPITATOR HAVING A POWER SUPPLY WHICH FURNISHES HIGH VOLTAGE TO THE DISCHARGE ELECTRODES RESPONSIVE TO OPERATING CONDITIONS WITHIN THE PRECIPITATOR AND A CONTROL MEANS FOR SEQUENCING RAPPER OPERATION, THE METHOD COMPRISING: DIRECTING THE FLOW OF GAS TO BE CLEANED THROUGH THE PRECIPITATOR, SUPPLYING HIGH VOLTAGE TO THE DISCHARGE ELECTRODES OF A MAGNITUDE SUFFICIENT TO CAUSE COLLECTION OF CHARGED PARTICLES ON THE COLLECTOR ELECTRODES, RAPPING THE ELECTRODES IN A TIMED CYCLE TO DISLODGE THE COLLECTED PARTICLES, COLLECTING THE DISLODGED PARTICLES, AND INTERRUPTING RAPPING CYCLE WHEN ADVERSE OPERATION CONDITIONS OCCUR WITHIN THE PRECIPITATOR TO REDUCE REINTRAINMENT IN THE GAS STREAM OF COLLECTED PARTICLES DURING SAID OCCURRENCES. 